The exemplary embodiment relates to the color management arts. It finds particular application in connection with a system and method for minimizing differences in color measurements from different instruments.
For the printing industry, many “off the shelf” spectrophotometers are available to perform color measurements. Numerous illumination-viewing geometries (e.g., 0/45, 45/0, integrated sphere) are available. The various devices may differ in speed of measurement, illumination size, illumination spectrum, detection resolution, chromatic sensitivity, and the like. Because of these differences, different devices differ in their spectral measurements with various media and colorants. Examples of physical properties that may affect the measurements include gloss, fluorescence, texture, motion of media, and media backing. Currently, the physical properties of the print media have to be narrowly specified in order for the color to be measured accurately to well within perceptual differences and to be repeatable by different instruments.
Recently developed color management applications often make use of an in-line spectrophotometer or similar color measurement device. Examples include tone reproduction curves (TRCs) generated using data from an in-line spectrophotometer during a color calibration routine. They are also used for accurate spot color matching, in which a specified spot color is to be printed by a process color device. For example, spot colors may be defined using the Pantone Matching System, Pantone GoE colors, user defined spot colors, or the like. Other applications include performing color check routines and generating accurate multidimensional custom ICC (International Color Consortium) profile look up tables (LUTs). The custom ICC profiling function lets the customer select a paper stock to profile and automatically print, measure, and generate the color profiles needed for various halftone screens. After generating the profiles, the customer can then proceed immediately into production, with the corrections applied, and receive more accurate output with minimal adjustments so that the output emulates the industry color standards, such as GRACoL, ISO, SWOP, and Japan Color.
Because color measurement devices have different responses, those applications which require accurate and repeatable color measurements are often referenced to some “golden measurement standard” device. Each color standard may specify a particular device (such as an X-Rite iSis or DTP70 Autoscan spectrophotometer) that is to be used to generate the reference target (aim) measurements. For example, in the Pantone Matching System, the device independent targets supplied by Pantone, Inc., are measured by their standard instrument, an iSis spectrophotometer. The source aim in the CMYK path that meets GRACoL® standards is a SWOP® CMYK to L*a*b* LUT, which is developed on an offset press using measurements from an X-Rite DTP70 sensor.
Some of the existing problems with color matching could be minimized if the industry were to adopt a common instrument as the standard for performing measurements. Even if this were to happen, however, it is not likely to be cost effective to put such devices into the print path of a printing system for regular calibration purposes. Hence, to provide good color matching to meet the customer's expectations, differences in instruments in the aim measurements need to be accounted for.
In-line spectrophotometers have been found to give different measurements when testing just-fused sheets from those when the printed sheets have been cooled. Mathematical methods have been developed to correct for these thermochromatic errors so that the in-line device can provide an estimate of the measurement which that spectrophotometer would provide at the ambient temperature at which the customer will perceive the printed colors. One method involves building a thermochromaticity compensation matrix that relates the thermochromatically shifted (hot) colors to thermochromatically stable (cool) colors. This matrix is then applied as a signal processing function to subsequent in-line color measurements, thus producing a final spectral measurement that closely approximates the stable (cool) color.